Take a moment to look at yourself in the mirror. I want you to really examine your features—the curves, lines and shapes that make up your face. How broad is your chin? Narrow, or wide? How big is your mouth in comparison? Or your nose? Do you have strong, prominent eyebrows? How close are they together?

Just look at this face. Do you trust me? A new study in PLoS ONE says you might, even though you don't know why.

Or, more simply, what color are your eyes?

In a study published today in PLoS ONE, researchers from from Charles University in the Czech Republic had 238 participants rate the faces of 80 students for trustworthiness, attractiveness, and dominance. Not surprisingly, they found that the three measures correlated well with each other, with faces rating high on one scale rating high on the other two. Female faces were generally more trustworthy than male ones. But that’s wasn’t all. A much more peculiar correlation was discovered as they looked at the data: brown-eyed faces were deemed more trustworthy than blue-eyed ones.

It didn’t matter if the judge was male or female, blue-eyed or brown-eyed. Even accounting for attractiveness and dominance, the result was the same: brown-eyed people’s faces were rated more trustworthy. There was some evidence of in-group bias, with blue-eyed female faces receiving lower ratings from brown-eyed women than from blue or green-eyed ones, but this difference didn’t drive the phenomenon. All the participants, no matter what eye color they had or how good-looking they thought the face was agreed that brown-eyed people just appear to look more reliable.

The real question is why? Is there a cultural bias towards brown eyes? Or does eye color really correlate somehow with personality traits like accountability and honesty? Does eye color really matter that much?

To find out, the scientists used computer manipulation to take the same faces but change their eye colors. Without changing traits other than hue of the iris, the researchers swapped the eye colors of the test faces from blue to brown and vice versa. This time, the opposite effect was found. Despite the strange correlation to eye color, the team found that eye color didn’t affect a photo’s trustworthiness rating. So it isn’t the eye color itself that really matters—something else about brown-eyed faces makes them seem more dependable.

To get at what’s really going on, the researchers took the faces and analyzed their shape. They looked at the distances between 72 facial landmarks, creating a grid-like representation of each face. For men, the answer was clear: differences in face shape explained the appeal of brown eyes.

Shape changes associated with eye color and perceived trustworthiness, from the grid-based facial shape analysis done by the researchers. Note the similarities between the shapes of brown-eyed faces and trustworthy ones.

“Brown-eyed individuals tend to be perceived as more trustworthy than blue-eyed ones,” explain the authors. “But it is not brown eyes that cause this perception. It is the facial morphology linked to brown eyes.”

Brown-eyed men, on average, have bigger mouths, broader chins, bigger noses, and more prominent eyebrows positioned closer to each other, while their blue-eyed brethren are characterized by more angular and prominent lower faces, longer chins, narrower mouths with downward pointing corners, smaller eyes, and more distant eyebrows. The differences associated with trustworthiness are also how our faces naturally express happiness—an upturned mouth, for example—which may explain why we trust people who innately have these traits.

Although the trend was the same for female faces, researchers didn’t find the same correlation between trustworthiness and face shape in women. This result is puzzling, but female faces were overall much less variable than male faces, so it’s possible the statistical analyses used to test for correlation were hampered by this. Or, it’s possible that something else is in play when it comes to the trustworthiness of female faces. The researchers hope that further research can shed light on this conundrum.

Given the importance of trust in human interactions, from friendships to business partnerships or even romance, these findings pose some interesting evolutionary questions. Why would certain face shapes seem more dangerous? Why would blue-eyed face shapes persist, even when they are not deemed as trustworthy? Are our behaviors linked to our bodies in ways we have yet to understand? There are no easy answers. Face shape and other morphological traits are partially based in genetics, but also partially to environmental factors like hormone levels in the womb during development. In seeking to understand how we perceive trust, we can learn more about the interplay between physiology and behavior as well as our own evolutionary history.

About the Author: Christie Wilcox is a science writer and blogger who moonlights as a PhD student in Cell and Molecular Biology at the University of Hawaii. Follow on Google+. Follow on Twitter @NerdyChristie.

8 Comments

The trusthfulness of a person comes surely from the inate ability to receive the first impressions without any judgement. That gives an unquestionable confiability spectral resemblance. And it seems an inate ability that some people really have. Some people are straightforward secure about their positions and so it seems they value others have the same posture. Maybe those characteristics have something to do with the way they have been raised, naturally with kindness as priority matter. About the color of the eye though, I can only infer that the color will change with another evolutionable matter being set as priority, if the research is right. And it seems reasonable to me…I’ve never seen cats with brown eyes!

As in so many such studies reporting universally applicable conclusions (I haven’t read the research report), they select a subject population from an inherently biased pool – of generally local undergraduate students. In this case, I’m presuming that the study subjects as well as the objects were students of Charles University in the Czech Republic, although it wasn’t specifically stated in this article.

So what this study most likely actually determined is that young adults from the general region of the Czech Republic whose principal occupation is currently student have correlate specific facial features with personality characteristics such as trustworthiness.

Especially given the fairly recent attempts at ‘ethnic cleansing’ in the region in which the study was conducted, I would not expect quite the same results for a subject population of Nigerian soldiers, Japanese chefs or retired NFL players, for example.

BTW, I should have more specifically mentioned a subject population drawn from a typically blue-eyed racial group, such as retired Scandinavian health care workers, for example, to best illustrate my point.

Anyone with any kind of common sense can look at this “scientific experiment” and tell that it is bias and very flawed. In this article, it was completely specified to me that in fact, attractiveness has nothing to do with eye color, it had to do with facial symmetry, and facial symmetry alone.

The correct way to conduct this experiment would have been to select both blue eyed and brown eyed individuals with good to poor facial symmetry. The fact that people rated the “brown eyed faces” more attractive has nothing to do with the fact that they had brown eyes, it related more to the fact that they have attractive facial symmetry.

This experiment is inherently bias because quite frankly it sounds very much like the brown eyed selected were more attractive on the scale of facial symmetry than their blue eyed counterparts.

This experiment has nothing to do with eyes, it only has to do with facial symmetry, which has been proven time and time again to correlate to trustworthiness and attractiveness.

“To find out, the scientists used computer manipulation to take the same faces but change their eye colors. Without changing traits like [the] hue of the iris, the researchers swapped the eye colors of the test faces from blue to brown and vice versa.”

What a silly statement: “Without changing traits like [the] hue of the iris.”

“Hue” is EXACTLY the scientific (physics) and electrical engineering term for the change in wavelengths that distinguish colors, such as the eye colors:
black, blue, brown, gray, green, violet, amber, etc.

In the definitions, “hue” means exactly the same as “tint”. Some electrical engineers, like me, have studied such things as color because that scientific knowledge was and is necessary for making color TV work right.

Also, “saturation” is important because this prescribes how much white has been added to a basic hue (tint). A saturated color hasn’t had any white added to it, but an unsaturated color has had white added to it: e.g. pink is red that has had white added to it, and aqua is blue-green that has had white added to it. We can make tens of thousands of unsatuated colors.

Also, “shade” specifies how much black has been added to the basic hue (tint), not. With no black, we have a pure hue, but with black added, we get tens of thousands of different kinds of gray, and other shades of various colors.

The word “tone” is absolutely meaningless concerning colors because it is never defined anywhere. Don’t use it. Then in the United States and other English-speaking countries, there are TV commercials (especially concerning cosmetics) in which they use the terms “hue”, “tone”, “tint”, and “shade” willy-nilly as if they are all synonymous with each other — but they really aren’t. This is because advertising agencies do not know anything about colors, and their employees are unwilling to do any Internet research on the subject to find out. (I call this “being intellectually lazy”.)

Whoever wrote this article for Scientific American ought to know what “hue” means, and that it is synonymous with “tint”, but obviously she was too lazy to go find out.

I do not see any reason for people to gripe about how they perceive “bias” or whatever in this report on research in psychology. It is clear that the scientists were reporting their preliminary results, and they were not reporting results “set on stone tablets”.
Once again, this is a report on early results, and it invites more research by psychologists, etc., around the world — and complaining about it is in very poor taste.
Their results were also examined by the well-educated scientific editors of the journal, and the report was examined by a board of independent experts in the field.
I would like for you to walk in the researchers’ shoes, and have people complain about everything that you say and do, including your preliminary results.

Dear Ms. Wilcox:
I apologize for addressing you harshly. Mea culpa.
It is just that so many writers (in so many sources) never pay any attention to the comments that are posted after their articles, and even if they do, they are not interested in receiving any corrections whatever. They simply do not respond no matter how glaring the error is. Shame on them.

Yes, there is a big difference between “without changing the hue” and “without changing anything else but the hue”.
I have been known to write down exactly the opposite of what I meant, from time to time.
I can only suggest that before you (or anyone else) publish anything on the Internet, have someone else (or better: two other people) read it first and look for errors.
In my case, I am an electrical engineer with a master’s degree, and I continually see massive errors in print concerning electricity, electronics, and telecommunications. (This happens on TV, too.) It is irritating, believe me!
The most common one is that most writers have no idea of the difference between voltage and current. It is “shocking”!